Aircraft engine acoustic liner and method of making the same

ABSTRACT

A middle layer ( 22 ) having partitioned cavities ( 28 ) is sandwiched between an imperforate sheet ( 24 ) and an optional perforate sheet ( 26 ) to form a low resistance acoustic liner ( 22 ). A preferred method of manufacture of the low resistance acoustic liner ( 20 ) uses aspects of the buried septum technique including placing an uncured imperforate septum ( 36 ) between a support layer ( 32 ) and a partitioned cavity middle layer ( 22 ), pressing the middle layer into the imperforate sheet ( 24 ) and through the support layer ( 32 ), curing the imperforate septum  36  to form the imperforate sheet ( 24 ), removing the support layer ( 32 ), and attaching a perforate sheet ( 26 ) of roughly at least 15% open area to the top side of the middle layer. A preferred embodiment of inner and outer linings ( 47 ), ( 49 ) is provided. Each lining includes a combination of annuluses of absorptive and/or low resistance liners ( 51 ), ( 53 ). A first arrangement of engine acoustic linings includes alternating sections of absorptive liner with low resistance acoustic liner, one after the other. The low resistance acoustic liner scatters low mode order noise into a range of noise orders, including both high and low mode order modes. The absorptive liner absorbs the high mode order noise and dissipates it as heat. A second arrangement of acoustic linings includes a splitter ( 70 ) having a portion formed of low resistance liner ( 72 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/079,677, filed May 15, 1998, now U.S. Pat. No. 6,209,679, which is adivisional of U.S. patent application Ser. No. 08/662,456, filed Jun.13, 1996, now U.S. Pat. No. 5,782,082, both of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to noise dissipation panels, and moreparticularly to acoustic liners and arrangements of liners in aircraftengines and surrounding surfaces.

BACKGROUND OF THE INVENTION

Aircraft engine noise is a significant problem in high population areasand other noise controlled environments. Attempts currently focus onlining the aircraft engine nacelle and surrounding engine areas withacoustic liners to reduce the amount of noise radiating to thecommunity.

As background information regarding general engine acoustic theory,there exists a linearized wave equation that describes the acousticpressure distributions present in an airflow duct. This wave equationhas a general solution given by a superposition (i.e., discretesummation) of eigenfunctions. Eigenfunctions vary with the boundaryconditions at the duct wall, i.e., the wall's impedance. There are aninfinite number of such eigenfunctions, each with an associatedeigenvalue, that are referred to as the “modes of propagation”, or“modes” for short. In general, low order modes have eigenvalues that arelow in absolute value. High order modes have eigenvalues that are highin absolute value. Typical low mode order values for aircraft enginenoise are 0 to 5, though the range will change depending on frequency,duct size, etc. Typical high mode order values for aircraft engine noiseare 8 to 15, though, these will also vary.

As used in the discussions below, the term “low mode order noise” ismeant to describe noise waves that are represented mathematically byrelatively low absolute value eigenvalues with respect to the range ofnoise modes present in a given application. When viewed physically, theorder of mode corresponds generally to the angle of wave propagation inthe duct relative to the duct walls. As shown in FIG. 1, engine noisewavefronts 12 propagate along the duct 13 at various angles θ relativeto the duct walls 14. If the angle is zero, the wave is said to be afundamental wave 15, i.e., θ=θ_(f)=0. Fundamental waves have wavefrontsthat travel in the axial direction of the duct and have a uniformpressure distribution at any particular duct cross-section.

In addition to fundamental waves, there are non-fundamental noisewavefronts which reflect back and forth between the duct walls as thewavefronts travel along the duct. These non-fundamental wavefrontscreate a non-uniform pressure distribution across the ductcross-section. The non-fundamental waves are generally classifiedaccording to their angular directions relative to the duct walls. Loworder modes of noise propagation have wavefronts 16 oriented at smallangles as measured relative to the duct walls, i.e.,θ=θ_(l)≦approximately 30 degrees. High order modes of noise propagationhave wavefronts 17 oriented at relatively larger angles as measured fromthe duct walls, i.e., θ=θ_(h)≧approximately 60 degrees. A wide range ofnoise frequencies exists for each mode order, low or high. For furtherdiscussion of theoretical considerations, see Aeroacoustics of FlightVehicles, by Harvey H. Hubbard, published for the Acoustical Society ofAmerica through the American Institute of Physics, 1995. See also,Theoretical Acoustics, by Philip M. Morse et al., McGraw-Hill BookCompany, dated 1968.

In a given aircraft application, an engine will generate both high andlow mode order noise. Current design practice focuses on reducing thisnoise through the use of absorptive acoustic liners. Absorptive linersare known in various configurations, including the use of a honeycombcore sandwiched between an imperforate sheet and a perforate sheethaving a small amount of open surface area. This particular combinationis sometimes referred to as a single degree of freedom absorptiveacoustic liner.

Absorptive liners are successful because pressure waves cause air topass into and out of the openings of the perforate sheet and toexperience a sufficient amount of friction, or resistance, which isdissipated as heat energy. The overall impedance of an acoustic liner isa complex number, given by a real part, the resistance, and an imaginarypart, the reactance. Resistance relates to the liner's ability todissipate noise energy as heat. Reactance relates to the liner'stendency to react noise energy back onto itself Absorptive linersprovide moderate resistance and low reactance for high mode order noisewaves.

FIG. 2 illustrates the theoretical effect of using absorptive liners fora hypothetical case. A given total noise energy 18 is initiallycomprised of a combination of one low and one high mode order noise 19,21, each having equal energy. Starting at the beginning of the duct atposition 0, the total noise energy 18 encounters an absorptive linerthat quickly reduces the high mode order noise 21 and more slowlyreduces the low mode order noise 19. Since high mode order noiseattenuates quickly in the duct, only a relatively short duct length isneeded to dissipate most of the high mode order noise present. In FIGS.2 and 3, the vertical axis is logarithmic. A change of about −3 dB, forexample, refers to a reduction in noise energy of about half Thehorizontal axis is normalized to be dimensionless. The exact values ofthe information shown in FIGS. 2 and 3 will vary according to thecharacteristics of a particular application, and in general there willbe energy in more than two modes.

As is evident by FIG. 2, absorptive liners are very effective forabsorbing high mode order noise 21, but are inefficient for reducing lowmode order noise 19, i.e., those noise wavefronts traveling along theduct at a low angular displacement relative to the duct walls.Propagating at low angles, these low order modes strike the absorptiveliners fewer times in a given length of duct. Therefore, to reduce allof the low mode order noise requires a greater length of acoustic liningthan is typically possible in the space-limited regions of aircraftengines. Noise reduction from use of absorptive liners is thuspractically limited to higher mode order noise.

Thus, a need exists for an acoustic liner, or arrangement of liners,that effectively reduces both high and low mode order noise. The presentinvention is directed to fulfilling this need.

SUMMARY OF THE INVENTION

The present invention provides a new type of acoustic liner andarrangement of liners specifically for use in dissipating low mode ordernoise. This new liner is termed a low resistance acoustic liner andincludes a middle layer, or core layer, having partitioned cavities. Thecavities aid in scattering a large amount of low mode order noise intohigher mode order noise. An imperforate sheet is attached to one side ofthe middle layer. A perforate sheet having a large open surface area isoptionally attached to the middle layer, opposite the imperforate sheet,so that the partitioned cavities of the middle layer are substantiallysandwiched between the imperforate and the perforate sheets. Theperforate sheet stops the whistling effect caused by high speed airflowing into the cavities and minimizes airflow drag as may be requiredfor some applications, e.g., in aircraft engines. The preferred middlelayer material for high-speed commercial aircraft engines is honeycombcore having cavities with axes preferably oriented perpendicular to thecentral plane of the core.

In accordance with further aspects of the invention, the preferredmiddle layer cavity depth is approximately one quarter the noisewavelength sought to be reduced. The middle layer cavities may also becomprised of cavities having varying depths. The preferred averagediameter of each middle layer cavity is equal to or less thanapproximately one tenth the noise wavelength. For noise havingwavelengths from 3 to 12 inches, the cavity depth is between roughly0.75 and 3 inches, and the cavity average diameter is between roughly0.3 to 1.2 inches.

In accordance with other aspects of the invention, the perforate sheetincludes uniformly distributed openings. The percent open area of theperforate sheet is preferably in an amount of at least 15% the entireperforate sheet surface area. These openings are preferably holes thathave diameters less than the average cavity diameter. The low resistanceacoustic liner has an absorption coefficient of about 0 to 0.5, thepreferred value being less than 0.5. The low resistance acoustic linerhas a resistance coefficient of about 0 ρc to 0.5 ρc, the preferredvalue being 0.3 ρc.

In accordance with yet further aspects of the invention, a preferredmethod of manufacturing a low resistance acoustic liner having cavitiesof constant depth includes attaching the imperforate sheet to one sideof the middle layer using an adhesive. The perforate sheet is attachedto the opposite side of the middle layer also using an adhesive.

In accordance with yet other aspects of the invention, a preferredmethod of manufacture for creating cavities of varying depth includesproviding a support layer of hardened wax-like material, placing anuncured imperforate septum on top of the support layer, placing a middlelayer formed of a partitioned cavity material (e.g., honeycomb core) ontop of the uncured imperforate septum, and using a uniform force topress the middle layer through the septum and the support layer untilthe septum is located at a desired cavity position. The septum is curedto form the imperforate sheet. After curing, the support layer isremoved, such as by melting, and a perforate sheet is optionallyattached to the side of the core opposite the cured septum.

In accordance with still further aspects of the invention, the desiredimperforate septum material is a thermoplastic, resin, rubber, orrubber-like film material with which the middle layer can be used to“cookie-cut” during pressing and that which can be later cured to form aseal within each cavity. The preferred support layer material is wax.The support layer is of varying cross-sectional thickness in order tocause the imperforate sheet to be located at different heights withinthe middle layer cavities. A mating layer may be optionally used to aidin supporting the middle layer as it is forced through the uncuredimperforate septum and support layer. The mating layer is placed betweenthe uncured imperforate septum and the middle layer prior to pressing.The surface shapes of the mating layer and the support layer arematched. The method of making a low resistance acoustic liner mayoptionally include the step of warming the support layer prior topressing so that the support layer may be more easily cut by the middlelayer.

In accordance with still other aspects of the invention, a firstembodiment of acoustic liners includes inner and outer linings formed ofalternating low resistance liners with absorptive liners. The liners arepositioned side-by-side and extend within an airflow duct for a lengthas necessary or as space allows. It is preferable that an absorptiveliner initially precede the first low resistance liner. The liners maybe formed to replace duct walls or may be attached directly to theexisting duct walls. All liners are placed with their imperforate sheetslocated farthest from the airflow. The low resistance acoustic linersurface area adjacent the airflow is sized approximately 5% to 25% thesize of the absorptive liners. For circular engine locations such as inthe bypass duct surrounding an turbofan engine core, the liners areformed as annuluses.

In accordance with yet additional aspects of the invention, a secondembodiment of acoustic liners includes inner and outer linings with asplitter positioned generally mid-way between the linings. The splittermay be formed entirely of low resistance acoustic liner or may formed asa combination of both low resistance liner and absorptive liner. Forcircular engine locations, it is preferable to use annular linersattached to surrounding engine structures using conventional attachmentmethods.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic side elevation illustrating noise wavepropagation in a duct;

FIG. 2 is a chart illustrating theoretical reduction in noise energyrelative to duct length when using absorptive acoustic liners;

FIG. 3 is a chart illustrating theoretical reduction in noise energyrelative to duct length when additionally using a low resistanceacoustic liner formed in accordance with the present invention;

FIG. 4 is a fragmentary top perspective of a low resistance acousticliner formed in accordance with the present invention;

FIG. 5 is a side elevation illustrating a method of forming a lowresistance acoustic liner formed in accordance with the presentinvention, including a bottom support layer, a deformable septum, and apartitioned core;

FIG. 6 is a side elevation of a first variation of the support layer ofFIG. 5;

FIG. 7 is a side elevation of a second variation of the support layer ofFIG. 5;

FIG. 8 is a side elevation of an alternative method of forming a supportlayer in accordance with the present invention, using mating layers andan imperforate septum;

FIG. 9 is a side elevation illustrating the method of forming a lowresistance acoustic liner of FIG. 5 utilizing the mating layers andimperforate septum of FIG. 8;

FIG. 10 is a side elevation of a portion of an aircraft engine withparts broken away for illustrative purposes, showing a first arrangementof acoustic liners formed in accordance with the present invention;

FIG. 11 is a side elevation of a portion of an aircraft engine withparts broken away for illustrative purposes, showing a secondarrangement of acoustic liners formed in accordance with the presentinvention;

FIG. 12 is a side detail illustration of a first alternative embodimentof a splitter formed in accordance with the present invention; and

FIG. 13 is a side detail illustration of a second alternative embodimentof a splitter formed accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The discussion below first focuses on a description of the theoryunderlying the present invention low resistance acoustic liner and adescription of its preferred configuration and method of manufacture.This is followed by a description of two particularly usefularrangements of liners for use with modern aircraft engines. Thesearrangements are generally constructed by placing alternating sectionsof absorptive liner with low resistance acoustic liner, one after theother, lengthwise within the engine. The low resistance acoustic linerscatters low mode order noise into a range of noise orders, includingsome high order modes. The adjacent downstream absorptive liner absorbsthe high mode order noise and dissipates it as heat. This arrangement oflow resistance acoustic liner followed by absorptive liner is repeatedas many times as space allows, or as necessary.

With regard to the underlying theory of how low resistance acousticliners work, it is our belief that low mode order noise waves react withthe low resistance acoustic liner and become scattered into both highand low mode order noise. The low resistance acoustic liner offersimpedance that includes a small real resistance term as well as a smallimaginary reactance term. Near zero impedance, or at least <0.5 ρc,appears to cause a portion of the low mode order noise to be scatteredinto high mode order noise (where ρ is the density of the air, and c isthe speed of sound in the air). This newly created high mode order noisemay then be absorbed by downstream absorptive acoustic liners.

Referring to theoretical FIG. 3, the total energy 18 is initiallyreduced primarily by a reduction in high mode order noise 21 from use ofa first absorptive liner. Upon encountering a low resistance acousticliner formed in accordance with the present invention, roughly half thelow order mode energy 19 is scattered into high mode order noise 21′,with half remaining as low mode order noise 19′. A second absorptiveliner absorbs the newly formed high mode order noise 19′. This processis continued as space allows. In this manner, the total noise energy is“ratcheted” down to values unattainable by use of absorptive acousticliners only.

FIG. 4 shows a low resistance acoustic liner 20 of the present inventionthat includes a middle layer, or core layer, 22 generally sandwichedbetween an imperforate sheet 24 and a perforate sheet 26. The middlelayer 22 is formed of a material having partitioned cavities 28 therein.Low mode order noise enters the middle layer cavities and exits as arange of modes. Thus, the cavities 28 “capture” large amounts of lowmode order noise and scatter it outward into both high and low modeorder noise. The low resistance acoustic liner 20 has an absorptioncoefficient of about 0 to 0.5, the preferred value being less than 0.5.The low resistance acoustic liner 20 has a resistance coefficient ofabout 0 ρc to 0.5 ρc, the preferred value being 0.3 ρc

The present invention may be practiced without a perforate sheet. Theperforate sheet 26 serves to stop the whistling effect caused by airthat would otherwise be flowing rapidly over and into the cavities. Theperforate sheet also serves as a smooth, flow-directing surface thatminimizes drag within the airflow duct from air interacting with the lowresistance liner. When a perforate sheet 26 is used, a plurality ofholes 30 should be formed therethrough. The percent open area of theperforate sheet 26 due to the holes 30 is preferably in the range ofabout 15-75% of the entire sheet surface area. The holes 30 are sizedless than the average diameter of each cavity. At these higher open areapercentages, it has been discovered that the perforate sheet stops thewhistling effect and minimizes airflow drag while still allowingsufficient amounts of low mode order noise to enter the cavities andexit in the form of both high and low mode order noise.

Known configurations of absorptive liners also include a perforate sheethaving a plurality of holes therethrough. See, for example, U.S. Pat.No. 4,433,021. Actual total open area amounts for absorptive linersvaries between approximately 5%-15%. Absorptive liners, however, rely onthe open area perforations to purposefully manipulate the noise wavepropagation in such a way as to encourage its dissipation. As statedabove, the present invention perforate sheet holes 30 are not concernedwith noise dissipation, but rather the avoidance of whistling andairflow drag.

Another type of prior art liner utilizes a layer of open weave to supplyfriction to noise waves for dissipation. See, for example, U.S. Pat. No.4,433,021. These liners sometimes use a perforate sheet to providestructural support for the layer of open weave. These liners includeperforate sheet layers with open areas between 15%-35%. Higher percentopen area values, however, are not desirable since the strength isdiminished as the open area increases. The present invention is notdirectly concerned with noise dissipation by absorptive liner alone, butis instead concerned with scattering noise while avoiding whistling andairflow drag in order to promote dissipation by an absorptive liner.

Still referring to FIG. 4, representative middle layer materials includehoneycomb core, Flexcore™, fluted core, etc. The preferred middle layermaterial for high-speed commercial aircraft engines is honeycomb corehaving partitioned cavities with axes substantially perpendicular to thegeneral plane of the core. A preferred cavity depth is approximately onequarter the wavelength of the noise sought to be reduced. This sizeprovides zero pressure at the cavity entrance and maximum particlevelocity at the imperforate sheet. To increase scattering for a givenfrequency or for applications having multiple frequencies present in thelow mode order modes, cavity depth may be made to vary from cavity tocavity. At least one type of variable depth cavity has been found toprovide superior results. This embodiment is discussed in detail belowwith reference to FIG. 6.

The optimum cavity width is relatively small as compared to thewavelength of the noise that is to be reduced, a nominal value being onetenth the wavelength. By way of example, for noise having wavelengthsfrom 3 to 12 inches, the cavity depth is between about 0.75 and 3inches, and the cavity average diameter is between about 0.3 to 1.2inches. As will be appreciated by those skilled in the art, the overallmiddle layer dimensions will thus vary according to the needs of aparticular application, the above dimensions being provided as examplesonly and not intended to be limiting.

The imperforate and perforate sheets 24, 26 may be formed by any one ofa number of known materials, e.g., aluminum, fiber glass, composites,etc. The perforate sheet 26 is attached to the middle layer 22 usingsuitable conventional methods, such as gluing. It is attached to thatside of the middle layer that is to be lying adjacent the engineairflow. The imperforate sheet 24 may be attached to the middle layer 22also using conventional methods. For example, an imperforate sheet maybe simply adhered to the middle layer using adhesives. The imperforatesheet 24 is positioned generally on, or near, that side of the middlelayer which is opposite the perforate sheet 26.

One method for creating cavities of varying depths is to simply cut themiddle layer surfaces in the desired contour prior to attachingimperforate and perforate sheets thereto. A preferred method forcreating cavities of varying depths, is by using the buried septumtechnique. This method is a preferred method because it can allow fordeeper middle layers that may provide a structural function, and thismethod can provide a solid connection of imperforate material to eachmiddle layer cavity. The buried septum acoustic liner manufacturingtechnique is disclosed in U.S. Pat. No. 4,257,998 and U.S. Pat. No.4,265,955, both incorporated herein by reference, to the extentconsistent with the present specification. This method generallyincludes providing a support layer of hardened wax-like material,placing an uncured perforated septum on top of the support layer,placing a partitioned cavity material (e.g., honeycomb core) on top ofthe uncured septum material, and pressing the core through the septumand support layer until the septum is located at a desired cavityposition. The septum is cured to cause it to adhere to the inner wallsof each cavity at its selected height without restricting the septumperforations. The support layer is removed, such as by melting. Aperforate sheet is glued to one side of the core, and an imperforatesheet is glued to the opposed core side. The result is, effectively, theformation of two perforated cavity layers from a single core. One layeris formed between the perforate sheet and the perforated septum, theother between the perforated septum and the imperforate sheet.

With regard to the present invention low resistance acoustic liner, analtered buried septum technique is used, as shown schematically in FIG.5. An uncured imperforate septum 36 is placed between a support layer 32and the partitioned cavity middle layer 22. A uniform force is providedto press the middle layer through the imperforate sheet 24 and into thesupport layer 32. The force is represented in FIGS. 5 and 9 as arrows34. After the uncured imperforate septum 36 is pushed to the desiredcavity location, the uncured imperforate septum 36 is cured to form theimperforate sheet 24 discussed above. Using this technique, theimperforate sheet 24 is easily and permanently adhered to the innerwalls of the middle layer cavities 28. The support layer 32 is removed,e.g., by melting. The present invention low resistance acoustic liner 20is optionally provided with a perforate sheet 26 having at least 15%open area. A preferred range of open area is between roughly 15%-75%.

Because there is no need to form two perforated cavity layers in the lowresistance acoustic liner (as there is in the absorptive liners of the'998 and '955 patents), the support layer 32 does not need to be verythick. The height of the support layer determines only the location atwhich the uncured imperforate septum 36 will adhere to a cavity wall.After the imperforate septum is cured, the portion of the middle layerextending beyond the septum may be used as support structure or used inattaching the low resistance acoustic liner to a surface. Alternatively,the extending portions may be removed entirely (e.g., via cutting,sanding, grinding, etc.)

In this manner, the buried septum technique is altered in order toprovide an efficient method of forming what is basically a bottomsurface to each low resistance acoustic liner middle layer cavity 28.This method for inserting an imperforate septum within the middle layermay be applied whenever the desired imperforate septum material ispreferably of a thermoplastic, resin, rubber, or rubber-like filmmaterial to which the middle layer 22 can be used to “cookie-cut” theseptum material during pressing and that which may be later cured toform a seal within each cavity 28. A preferred support layer 32 is wax.

In the embodiment shown in FIG. 6, the cross-sectional shape of thesupport layer 32 is a simple wedge. The cross-sectional shape of thesupport layer 32 of FIG. 7 is a more complicated contour includinginclinations, declinations, concave curves, and convex curves. Wheninserted into the middle layer 22, the imperforate sheet 24 will belocated according to the height of the support layer cross-section atthat position. The present invention encompasses using virtually anyconceivable cross-sectional shape. The optimal shape will depend on thedesign considerations of a particular application.

The scattering efficiency of low resistance acoustic liners is afunction of whether the proper liner cavity depth is present for aparticular frequency. Because there are multiple frequencies in low modeorder engine noise, the imperforate sheet should be preferably attachedto the middle core at varying heights in order to enhance the range offrequencies affected and the modes scattered by the liner. The optimumsupport layer shape is a wedge, as shown in FIG. 6. The preferred wedgeangle of inclination is between about 30 to 60 degrees. The wedge shouldultimately be oriented in the duct such that cavity depth decreases inthe direction of noise propagation. This wedge cavity depth variationalso enhances scattering relative to the ¼ λ constant cavity depth linerat a single frequency. This is apparently because the wedge cavity actsas a better “reflector” at scattering noise into higher mode order thanoccurs without the wedge shape.

Minor distortions of the septum and/or cavities because of non-uniformstresses during pressing are not critical to the acoustic performance ofthe low resistance acoustic liner. If more significant forces ordistortions are present, as would be the case for a support layer havinga very complex cross-sectional shape, the above method may be furtheraltered as shown in FIGS. 8 and 9. In particular, after placing anuncured imperforate septum 36 on top of the support layer 32, a matinglayer 32′ is placed on top of the uncured imperforate septum 36. Thus,the imperforate septum is held firmly between the mated layers 32 and32′. Pressing and curing is done as usual, with the middle layer 22 corebeing inserted from the top surface of the mating layer 32′, asillustrated in FIG. 9. The step of removing the support layer is alsoaccomplished as described above, with the addition of removing themating layer as well. As will be readily appreciated by those skilled inthe art, this alternative method of construction provides additionalstructural support for the septum 36 and middle layer 22 as the middlelayer is forced through the layers 32, 32′ and the uncured imperforateseptum 36.

FIG. 10 is a side elevation of a portion of an aircraft engine withparts broken away for illustrative purposes, showing an arrangement ofacoustic liners positioned circumferentially about an engine core 46 ofa turbofan engine within a bypass duct 44 defined generally by an outerstructure 50, such as a fairing or nacelle, and an engine casing 52. Thearrangement generally includes inner and outer linings 47, 49. Thelinings of FIG. 10 include alternating annuluses, or rings, ofabsorptive liner 51 and low resistance liner 53 preferably connectedend-to-end using conventional methods, e.g., gluing, connecting withbrackets, etc. The creation of alternating absorptive and low resistanceliners may alternatively be constructed by using a single liner havingdistinct regions of low open surface area (to act as the absorptiveliner) and high open surface area (to act as the low resistance liner).Each low resistance acoustic liner annulus 53 scatters low mode ordernoise into a range of noise orders, including some high order modes. Thehigh mode order noise is absorbed by adjacent absorptive liners 51. Thelow resistance liners are formed using the methods described above. Theannuluses may be formed either as a single piece or as a combination ofjoined arcs, wherein each arc is formed separately. Other methods offorming the linings 47, 49 may be used, such as attaching the linersdirectly to a single annular backing. This method is not preferredbecause it adds unnecessary weight.

The inner and outer linings 47, 49 may be formed to replace an existingduct wall, may be attached directly to an existing duct wall, or may beconnected between stationary structures to define a new duct wall. Shownin FIG. 10, the outer lining 49 defines a duct wall where nonepreviously existed, while the inner lining 47 replaces a portion of thepre-existing engine casing 52. Shown in FIG. 10, the forwardmost edge ofthe outer lining 49 is connected to a radial flange 54 usingconventional techniques, e.g., fasteners, brackets, etc. The aftmostedge of the outer lining 49 is connected directly to the outer structure50. The inner lining 47 connects to the remaining portion of the casing52.

Still referring to FIG. 10, it is recommended to use an absorptive lineras the first liner encountered relative to the direction of noisepropagation. Such an initial absorptive liner immediately absorbs aquantity of high mode order noise, leaving the downstream liners toaddress attenuation of low mode order noise. As many alternating linersare provided as necessary, or until the aft end of the engine nacelle isreached. All liners are placed with their perforate sheet facing theregion of airflow. The preferred liner surface area lying adjacent tothe airflow is such that the annulus of low resistance acoustic liner 53is actually shorter in length than its downstream adjacent annulus ofabsorptive liner 51. A range of acceptable ratios is between about 5% to1% (i.e., the low resistance liner surface area is about 5-25% that ofthe absorptive liner surface area), with 15% being generally sufficient.This is because low resistance acoustic liners are needed to scatter lowmode order noise modes, which requires a relatively short distance. Theabsorptive liners, that actually perform the task of dissipating thehigher order noise modes, require relatively more distance.

FIG. 11 is a side elevation of a portion of an aircraft engine withparts broken away for illustrative purposes, showing a secondarrangement of acoustic liners, also located within the bypass region ofa turbofan engine. The inner and outer linings 47, 49 of FIG. 10 areformed entirely of an annulus of absorptive liner 51. A splitter 70having at least one portion formed from low resistance liner isincluded. For this arrangement, it is recommended to place a portion ofthe inner and outer linings forward of the splitter 70. In this way,high mode order noise is initially absorbed by the inner and outerlinings leaving the remaining arrangement to reduce low mode ordernoise.

The splitter 70 should include aerodynamically efficient leading andtrailing edges 76, 78, and should be relatively short in length ascompared with the overall duct length. The splitter 70 is preferablyannular and is positioned mid-way between the outer and inner linings inorder to balance the noise lessening capabilities between opposedsplitter sides. Conventional components are used to secure the splitterto a stationary engine structure. In FIG. 11, a plurality of radialbrackets 66 connects the splitter 70 to the outer structure 50 byextending through openings in the outer lining 49 and bolting to theouter structure. Any one of a number of known splitter attachmentmethods may be used.

The splitter of FIG. 11 includes a first section formed of two lowresistance liners 72 positioned back-to-back such that their imperforatesheets are adjacent one another. In FIG. 11, line 80 indicates thegeneral location of the adjacent imperforate sheets. For weight savings,it is recommended that a single imperforate sheet be used and sharedbetween the adjacent low resistance liner annuluses 72. The lowresistance liners 72 of the splitter 70 precede a section of absorptiveliners 74. This arrangement of low resistance liner with absorptiveliner may be repeated.

FIG. 12 is a side detail illustration of a first alternative annularsplitter 70′ formed entirely of low resistance liner material 72′. Thereare two low resistance liner annuluses shown in FIG. 12, though, adifferent number may be used. Line 80′ indicates the general location ofthe adjacent (or shared) imperforate sheets. FIG. 13 is a side detailillustration of a second alternative of an annular splitter 70″ formedentirely of low resistance liner having cavities with varying depths.There are four separate low resistance liners shown in FIG. 13, thoughagain, a different number may be used. Line 80″ indicates a linearinclination to each imperforate sheet, thus providing cavities ofvarious depths.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.For example, the present invention low resistance liner may be used invarious engine locations, not just in the bypass duct of a turbofanengine. Likewise, the basic arrangement of providing low resistanceliner to scatter low mode order noise, with absorptive liner to absorbhigh mode order noise, may be used in non-engine applications. Inaddition, the configuration aspects described above with regard to FIGS.10-13 may be combined in various ways to suit a particular application.

What is claimed is:
 1. A turbofan aircraft engine airflow duct having atleast one acoustic liner material therein adapted for attenuating enginenoise, wherein at least a portion of the liner is comprised of annularrings of at least two different materials, that are positioned in theairflow duct so that they alternate with respect to each other in thedirection of the air flow, the materials including: (i) an absorptivematerial adapted to absorb high mode order noise, and (ii) and amulti-layer low-resistance acoustic material adapted to scatter low modeorder noise and having a resistance coefficient in the range of from 0to 0.5 ρc, comprising a layer having a plurality of partitionedcavities, between an imperforate sheet and a perforate sheet, positionedin the airflow duct so that the perforate sheet is exposed to the airflow.
 2. The airflow duct of claim 1 wherein the ratio of the surfacearea of low-resistance material to absorptive material, along the innerperiphery of the liners in the duct, is in the range of from 1:20 to1:4.
 3. The airflow duct of claim 1 wherein the perforate sheet has anopen area of at least 15 percent.
 4. The airflow duct of claim 1 whereinthe partitioned cavities are of a length approximately one quarter thewavelength of the engine low mode order noise.
 5. The airflow duct ofclaim 1 wherein the partitioned cavities are of a length approximatelyone tenth the wavelength of the engine low mode order noise.
 6. Theairflow duct of claim 1 wherein the layer having a plurality ofpartitioned cavities is in the approximate shape of a wedge having anangle of from 30-60 degrees, and the low-resistance material ispositioned in the airduct so that cavity depth decreases in thedirection of noise propagation.
 7. A jet aircraft engine airflow ducthaving at least one acoustic liner material therein adapted forattenuating engine noise, wherein: (i) at least a portion of the lineris comprised of an absorptive material adapted to absorb high mode ordernoise, and (ii) a splitter having an elongated shape is positionedapproximately in the middle of the duct, parallel to the direction ofthe air flow, and has a first and second section, the first sectioncomprises (a) an absorptive material adapted to absorb high mode ordernoise, and is positioned downstream of (b) a multi-layer low-resistanceacoustic material adapted to scatter low mode order noise and having aresistance coefficient in the range of from 0 to 0.5 ρc, comprising alayer having a plurality of partitioned cavities, between an imperforatesheet and a perforate sheet, positioned in the airflow duct so that theperforate sheet is exposed to the air flow.
 8. The airflow duct of claim7 wherein the perforate sheet has an open area of at least 15 percent.9. The airflow duct of claim 7 wherein the partitioned cavities are of alength approximately one quarter the wavelength of the engine low modeorder noise.
 10. The airflow duct of claim 7 wherein the partitionedcavities are of a length approximately one tenth the wavelength of theengine low mode order noise.
 11. The airflow duct of claim 7 wherein thelayer having a plurality of partitioned cavities is in the approximateshape of a wedge having an angle of from 30-60 degrees, and thelow-resistance material is positioned in the airduct so that cavitydepth decreases in the direction of noise propagation.